1. Field of the Invention
The present invention relates to a Flow Control Device (FCD) that is unique in its ability to control low flow rate fluid flow to within very closely controlled and consistently maintained limits using etched disc technology.
2. Description of the Related Art
New aerospace systems are being designed and built to make use of ultra pure Xenon gas for propulsion and ionization control applications. These applications require very fine and consistent control of Xenon gas flow. Either too much or too little flow could cause failure to attain the mission goals. Excessive flow would also deplete the available Xenon gas supply so as to degrade the mission life.
A Flow Control Device (FCD) designed for use in this environment must satisfy a number of criteria. For example, the extreme sensitivity to contamination of high purity flow requires that the FCD be ultra clean and not generate contamination. This is to prevent the potential for contaminant particles left in the FCD during manufacturing, or generated by the FCD from entering the fluid flow. Moreover, the need for very tight flow tolerances requires that the FCD have the ability to make fine and coarse flow adjustments during manufacturing in order to economically meet the flow requirements.
The FCD must also address several potential problems. For example, the potential for degradation of flow due to contaminants upstream in the system in which the FCD is installed; the potential for damage due to over pressure, or to environmental loads such as vibration and mechanical shock which might lead to brittle material failure or other structural failure; and the limited potential for the real time fine tuning and/or course flow adjustments during applied FCD life through the control of electrical, mechanical, or other environmental inputs, must all be taken into account.
Existing flow control technologies do not sufficiently address one or more of the criteria/problems identified above. More particularly, existing, very low flow, flow controllers involve single orifice designs, designs with single or a few long tortuous paths, and sintered material plugs with multiple random tortuous paths. Each of these is lacking in some significant way.
An orifice design is difficult to control precisely, and is subject to operational degradation due to contaminants. A single, very small hole is easily plugged or restricted by a single contaminant particle. Also, very small variations in edge, surface, or size conditions will have a significant impact on actual flow.
A design based on a single (or a few) tortuous paths tends to be an improvement over a standard orifice, but still has the same control and flow degradation problems to a lesser degree. A tortuous path will also allow contaminant particles to be trapped during manufacturing. These particles are then difficult to remove in cleaning operations, but may tend to be released randomly during mission life or in a vibration environment.
A sintered plug design tends to be difficult to adjust precisely in manufacturing, and therefore requires excessive testing and sorting of product to get the desired flow adjustment. Also, the inherently non-clean manufacturing process of sintering tiny particles into a single piece makes good initial cleanliness impossible, and fine particles trapped in the sintered media may be released randomly during mission life or in a vibration environment. Fine particle system contaminant may tend to deposit on the plug surface and degrade flow over the mission life. Also, the sintered construction tends to be relatively brittle, and may lead to structural failure under mechanical loads.
Existing flow controlling etched disc designs are largely limited to flow filtration or relatively gross flow rate controlling applications. Existing etched disc flow controllers tend to benefit by reducing pressure drop across the disc. Also, in fluid filtration applications, a shorter path length is preferable for increasing flow capability through the disc path. However, excessively high flow through a given flow path of a standard configuration disc makes it impossible to attain sufficiently low and well controlled flows.
Thus, standard existing disc designs tend to try to maximizing flow and minimizing pressure drop. This and other design goals in existing disc design work against the ability to provide very low flow rate control. Accordingly, existing etched disc designs do not allow for sufficiently high and well controlled pressure drop of fluid passing from the outside to the inside perimeter of a disc when stacked face to face and compressed.
Moreover, existing disc material combination designs do not allow for sufficiently low leakage across the non-etched disc surfaces between mating discs. Excessive leakage across the non-etched disc surfaces between mating discs make it impossible to attain sufficiently low and well controlled flows. In that regard, standard existing disc designs typically make use of stainless steel or titanium materials. These do not have the necessary compliance and available surface finish to create an efficient face to face seal under appropriate compression loading.